1. Field of the Invention
The present invention relates to a wireless or wire communication, and in particular to a spread spectrum communication system using direct sequence spreading and a demodulator used in sampling received signals of the spread spectrum communication system.
2. Related Art and Other Considerations
Various modulation systems have conventionally been used for wireless data communications. Among them, the spread spectrum communication system has attracted attention.
The modulation systems which are commonly used for data communications are narrow-band width modulation systems which can be implemented by a relatively small circuit. These systems have a drawback of being sensitive to the multi-path or narrowband non-white noise indoors (in offices or factories and the like).
In contrast, the spread spectrum communication system is advantageous in that these drawbacks can be overcome by spreading the transmission spectrum of signals for carrying data with a spread-code (code used for spreading) so that the signals are transmitted at a broad-band width. Among the spread spectrum transmitting systems, the direct sequence (DS) spread system has been put into practice in some areas.
An example configuration of the direct spread spectrum communication system is described with reference to FIG. 1 and FIG. 2. FIGS. 1 and 2 show an exemplary configuration and are block diagrams illustrating the transmission and reception sides of a communication device used for the Direct Sequence Spread Spectrum DSSS communication system, respectively. This example shows a communication device for demodulating the signals which have been modulated by BPSK modulation techniques and transmitted.
On the transmission side, data to be transmitted from a data generating unit 101 is spread in a spreading unit 102 by using spreading codes which are generated by a spread-code generating unit 102c. The data is spread by a factor of k wherein k denotes the length of the spread-code. The data which was spread is referred to as "spread data". The unit of data to be transmitted is referred to as "one bit" and the unit of spread data is referred to as "one chip". In other words, one bit of transmitted data comprises k chips. Thereafter, the spread signal modulates an oscillation signal from a radio frequency (RF) oscillator 103s in a modulating unit 103 and is transmitted from an antenna via a radio frequency circuit (not shown).
On the receiving side, the transmitted signal is received by an antenna and is fed to a radio frequency circuit for converting it into an intermediate frequency (IF) signal. In the receiver, the received IF signal (see FIG. 2) is divided by a dividing-by-two splitter 104 into two signals which are input to respective multipliers 105 and 106. The two signals are converted into I and Q components of a base band signal by using cosine and sine components of a local signal from a local signal oscillator (VCO) 107. Alternatively, the conversion may be done by using a signal line and a phase-shifter for shifting one of the components of the local signal by 90 degrees.
The I and Q components of the base band are then sampled for quantization by analog to digital A/D convertors 108 and 109, respectively so that they are converted into digital data. The data are input into digital correlators 110 and 111 for establishing correlations for the respective data. The respective correlation outputs are fed to a timing detector 112 and latch circuits 113 and 114. The timing detector detects correlation spikes and causes respective correlation outputs to be latched in a timed relationship with the detected correlation spikes so that they can be demodulated by demodulator 115 to provide data.
In a receiver having the configuration of FIG. 2, sampling of plural samples such as two or three samples per one chip is conducted independently of the data and/or chip. This is due to the fact that a carrier to noise (C/N) ratio prior to despreading is very low and it is hard to conduct clock regeneration before despreading in the spread spectrum communication.
The correlators establish a correlation relationship by using all values of samples. Accordingly, a correlation is determined in the digital correlator by using values of 2k samples with two samples to one chip. The manner of sampling in the base band in this case is illustrated in FIG. 3A-FIG. 3D with reference to an eye pattern (FIG. 3A) based on a chip. Inherently, the eye is widest at the midpoint of the eye pattern and decreases its size as it becomes closer to the opposite ends thereof. The theoretical value of the error rate in the digital communication is generally a value which is measured at the midpoint of the eye pattern (refer to FIG. 3B).
An example in which two samples are sampled per chip is shown in FIG. 3A-FIG. 3D. A timing relationship A of FIG. 3C shows that the two samples (a) and (b) are substantially equally sampled with respect to the eye pattern. Although the eye is slightly smaller in amplitude than that at the midpoint in this case, both samples are valid. On the other hand, in the timing relationship B of FIG. 3D, sampling at sampling point (c) is carried out for the midpoint of the eye pattern so the amplitude is larger. Sampling at point (d) however, is carried out at a transition point of the chip, the value of the sampling point (d) being almost invalid.
Therefore, a problem occurs in the prior art that the sampling characteristics may deteriorate depending upon the sampling timing relationship if two samples are sampled per chip and independently of the timing relationship with the chip.
In accordance with the present invention, data in a communication system is serial/parallel-converted and spread in accordance with the same spreading code and multiplexed for communication. In this inventive system, spread signals are delayed and multiplexed for high rate transmission in a limited band. This inventive system uses a "delayed multiplexing technique". This system enables high rate transmission to be conducted in a limited band. Communication of data at 4 and 10 MBPS is possible when two and five signals are multiplexed, respectively.
An exemplary configuration of a transmitting system for a delayed multiplexing technique of the prior art is shown in FIG. 4. The data which is generated in a data generating unit 121 are coded as differential-valued data by a differential coding unit 122 and then converted into a number of parallel signals, the number of which is equal to the number of signals to be multiplexed by a serial to parallel (S/P) converting unit 123. The converted parallel signals are multiplied by a pseudo-random noise (PN) code from a PN generator 125 in multipliers 124-1 through 124-5 for spreading.
Subsequently, the spread signals are delayed by delay elements 126-1 through 126-5, respectively, and are mixed by a mixer 127 to provide a multi-valued digital signal, which modulates an oscillation signal from an RF oscillator 129 in a modulator 128. The modulated signal is frequency-converted by a frequency converting unit 130 and is transmitted via a power amplifying unit 131, etc.
High rate data communication is made possible by receiving and demodulating the signals which are multiplexed on the transmitting side of the communication system which has performed delayed multiplexing in such a manner as shown in FIG. 4.
An eye pattern for the delayed multiplexing system of FIG. 4 is shown in FIG. 5.
Since the signals to be transmitted are multi-valued signals as shown in an eye pattern in the delayed multiplexing system described herein, the error becomes larger, in comparison to errors of other points, if the samples are sampled at the transition points of the data.
If the normalized value of the eye pattern is assumed to be 1, the value of the eye pattern only changes in the range of 1 to -1 when multiplexing is conducted. The eye pattern only changes in the range of 1 to 0 at the transition position of the data. If multiplexing is conducted, the value of the eye pattern changes in the range of 1 to 5 as shown in FIG. 5. In this case, the value at the transition position changes from 1 to 3, causing substantial deterioration.
In order to avoid this deterioration, increasing the number of samples may be envisaged. This may be applied to the multiplexing configuration as shown in FIG. 4 as well as general code division multiple access (CDMA) communication in which multiplexing is conducted by spreading signals with different codes.
If there is one sample at the transition position of data when three samples are sampled to one chip, two of the samples would be valid. When three samples are sampled to one chip, two thirds of the samples would be valid. An improvement in characteristics can be correspondingly expected. If four samples are sampled to one chip, three fourths of the samples would be valid.
However, another problem may occur in that the power consumption will increase due to an increased number of samples.
An example configuration of a prior art correlator relying upon a plurality of samples is illustrated in FIG. 6. In FIG. 6, a reference numeral 116 denotes shift registers; 117 a replica-generating unit having spreading code data; 118-1 to 118-n are multipliers; 119 is an adder for adding the outputs from the multipliers 118-1 to 118-n. The correlator has, on its input side, shift registers 116, the number of which is equal to the number of samples for holding data of an input signal. Since the shift registers 116 require the circuits, the number of which is equal to the number of input signals for each of samples (for example, three and four circuits when one chip comprises three and four samples, respectively), its power consumption is high. Since most circuits comprise shift registers in the correlator, the power consumption for two samples is substantially half that for four samples.